Kate donned the iconic navy wrap dress by Issa London in 2010, when she and William delivered news of their wedding to media at St James's Palace.
The then Miss Middleton looked gorgeous in the elegant silk dress, which featured a pleated front, flared skirt and waist ties.
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Kate and Prince William got engaged while on holiday in Kenya
Also known as the Phylis, the item is available to buy online and in store at Harvey Nichols for £575.
Issa London and the Knightsbridge-based department announced the news on Twitter, sharing a photo of the chic number alongside the caption: "First seen on the Duchess of Cambridge, @IssaLondon's Phylis dress tops our wish lists."
Kate, 33, was catapulted into the limelight when she first started dating Prince William. The pair met while studying at the University of St Andrews together, when Kate read history of art and the future King took up geography.
Kate's navy blue engagement dress is on sale for £575
After nearly eight years William, also 33, popped the question during a romantic holiday in Kenya, and one month later in November 2010, the pair confirmed they were to marry.
It was after the big announcement that the "Kate effect" really took off, with fans rushing to copy the brunette beauty's style, even before she had become a member of the royal family. Brands such as Beulah London, L.K.Bennett and Alexander McQueen have all benefitted from the Duchess wearing their clothing.
Kate's first-born Prince George is also fast becoming a trend-setter. The youngster, who turns two this week, is often kitted out in adorable outfits, which, like his mum Kate's clothing, sells out in a minute.